Directional coupler



Feb. 13, 1951 J. T. BANG-ERT x-:TAL

DIRECTIONAL COUPLER 2 Sheets-Sheet l Filed Sept. 18, 1947 m 1l mi .NUQDQW MASS V. T E m M WN m Af LA B0 ./.R J6. nf S MVM B m. V w,

Feb. 13, 1951 J. T. BANGERT ETAL DIRECTIONAL COUPLER 2 Sheets-Sheet 2.

Filled Sept. 18, 1947 @GEES .Ser A l NN 8m 8m o8 ovmoow ce en om ow .o

v UP* OLLYH AJMLLJSUIU-a J. r BAA/GERT NW2/fps s. ROSEN W (/I l A7 7ORNEV Patented Feb. 13, 1,951

- nIREcTIoNAnCoUPLER John T. Bangert, Orange, N; J., andSidney Rosen, Los Angeles, Calif.,'assignors to Bell Telephone Laboratories, Incorporated, New K corporation of New York" f Y York, N. Y., a

Application September 18, 194'?, nSerial No. 774,828

.6 Claims. (Cl, 178-44) Thisinvention relates to `Wave transmission and more particularly to directional couplers.

The' principalobjectof the inventionisto increase the `frequency bandv over Which'- a loW directivity ratio is obtainable VinV a directional coupler. Another object vis to reduce the direct loss. "Other objects are to simplify the construction 'of such a coupler and to reduce the 'manufacturing cost. 1 y 1" A directional coupler isa device associated with atransmission line .for sampling separately the forward: and the'backwardY Waves inthe line. In its simplest form such a device comprises'an auxiliary transmission line with Itwo equal coupling links tothemain transmission line spaced a quarter Awavelength apart. .`The currents flowing in onedirection in the auxiliary line will be additive forlwaves propagatedin one `direction in the main -line but will cancel for Waves travelingvin the opposite direction. The transmission-lines may bev open wire lines, coaxial lines, or `-wave guides and the coupling links may be probes, loopsor apertures. T y

In order to measure accurately backward waves of small amplitude a smalldirectivity ratio is ref quired `forthe directional coupler. However, in a" two-link coupler this ratio is satisfactorilyy small .over onlyY a narrow band of frequencies. To make the directional coupler operable over a wider band th'einumber of 'couplinglinks may be increased. 'For example, in afpaper entitled Directional Couplers published -in thePr'oceed" ings of the I. R. E., Vol. 35, No". 2,' pages' 1601110y 165, February' 1947,W. WI Mumfordfdescribes directional couplersl which employ more than Atwo coupling links. The links have equal'spaoifngs of a Y quarter wavelength lbut their transmission factors are tapered according to the coeflicients of the binomial expansion. A i Y In accordance with the present invention the frequency band of a directionalcoupler is widened byemploying a. pluralityofpairs of substantially equal coupling links, withthe-displacement A between pairs greater than theintra-pair spacing B/between the linksjprminga pair. For a minimum directivity ratio throughout the band the displacement -factor R, r`which isthev ratio of `A` to 2B, is approximately unity., However, the' band may be further widened by reducing' this ratio. For an 'eight-link'c'oupler' an optimum' ratio is-approximately- 0.92.

When the Ydirectional couplencomprises Wave" guides Yconnected bycouplingholesg'it is easier to achieve a-lowvalue of direct loss with the multipleepair type ot-couplertham withr one'of 55 the binominal-coeliicient type having thesame number of holes. is a function of the total area of the holes.Y In the multiplepair type of coupler all of the holes may have a diameter as large as the width of the guide but in theV binominal coupler only the: central hole may be this large. I

In practice it has been'foundthat a number of' equal coupling links are easier to construct than a number of links in which the transmission. factor is required to be tapered in some manner from link to link. rFhe simpler construction reduces the cost of manufacture and makes it easierl to build a directional coupler to t given specications. Y f

The nature of the invention will be more fully understood from the following detailed description and by reference to the accompanying drawing,l in which like'reference charactersvreferto similar or' corresponding parts and invvhich:

, Figul shows a transmission system incorporating a four-link directional coupler;

Fig. 2 is a cross-sectional View of the directional coupler of Fig. l taken along the line 2 2; s Y lFig.y 3 shows the central fragment of the directional coupler of Fig. 1 modified in accordance with the invention to employ eight coupling links; Fig. 4 is a curve showing the directivity ratio D for the four-link coupler of Fig., Y1 plotted against the intra-pair phase rshiftangle ,B over a range of 360 degrees;

,Fig. 5 presents a familyof curves showing the D` characteristic of therfour-link coupler for three values of the displacement factor R;

1 Fig. 6 is the D-' characteristic of the eightvlink ycoupler of Fig. 3 plotted over a 36u-degree range; and Fig, 7 gives the lli-,B characteristics ofl .the eight-link coupler Vfor two values of R.`

`Inthe fourlin'k directional coupler shown in Figs. `1 and 2 electromagnetic waves fromhthe source I are vfed to a'load 2 of-arbitrary impedance Z1. through a main transmission line 3 of character-istic impedance Zo. An auxiliary transmission line also of characteristicimpedance Zo, is longitudinally juxtaposed to the line 3 -for Aa portion of its'length. The une are' terminatedjat one'nd in a load 5 and at the other `end Aina load teach of impedance Zo. As shown more-clearly in Fig. 2 the lines 3 and l are wave` guides` of. rectangular cross-section and OneofthenarrOWe-r sides 1 is common to both. Theguides.v3..and 4are electrically connected by two pairs of coupling links,l

same fdianeteroiri tli'e `com m on`wall V1.

This is because the direct loss y Fig. 3 shows a modified form of the directional coupler of Figs. 1 and 2 in which the two pairs of coupling links are increased to four pairs, constituted by the holes I2 to I9 in the common wall l. In other respects the eight-hole coupler is the same as the one shown in Figs. l and 2.

If the impedance ZL of the load 2 does not match the impedance Zu of the line 3, part of the forward wave from the source I is reflected as a backward wave traveling toward the left. In an ideal directional coupler the portion of the, forward Iwave which enters the auxiliary line 4 through the holes in the wall l will all be dis-1y sipated in the load 5 and none will reach the load 5. Also, the entire portion of the backward wave which enters the line i will be dissipated in the load 5, with none reaching thev load 6. If, for example, the loads 5 and 6 arepower-indicating devices the power associated with each of these waves may be separately measured.

The accuracy of such measurements depends upon a characteristic of the coupler which may be termed its directivity ratio D. This is defined as; the square root of the ratio of the power received at the load 5 to the power received at the load 6 when power is introduced into the line 3 from the source I, assuming that the lines 3 and, 4

are perfectly terminated. In an ideal coupler D is Zero.

An expression for evaluating the directivity ratio D for the four-hole coupler of Figs. 1 and 2 will nowV be presented, based upon the following additional assumptions:

l. The energy in the auxiliary line is small compare-d with that in the main line 3.

2. The phase shifts in the coupling holes 8 to I l are identical.

3. The coupling holes 8 to I I have the same orifice transmission factor T, defined as the ampli.- tude ratio of transmission therethrough.

4;. The lines 3 and i are lossless and have the same phase velocity.

With these, assumptions D=1/2 l [cos en -l-cos 017-2T(sin etz-{- 3- sin eb) -I-7`6T(cos @Irl-cos bl] l, (l)

where the bars indicate that the absolute magnitudes tarkerl.y

and Ag is the wave guide wavelength. As showny where A is the free space wavelength` and, as;

shown in Figs. l` and 2.

d=diameter of hole h =wave guide height t=thickness of the walll w=wave guide width Formula 6 is based on the transmission through an orice with, a. thin. wall plus me, loss in, a. cir.-

The factor T may be found 4 cular wave guide of length t operating below cutoff in the 'IEn mode. It has been found to check experimental data to within two decibels.

Fig. 4 is a computed characteristic showing the directivity ratio D plotted against the intra-pair phase shift angle over a complete angular cycle, for the four-,hole directional coupler of Figs. 1 andr 2, using Formula (1). The displacement factor R is unity, which, in accordance with Equation 4, means that the displacement A is equal to twice the intra-pair spacing B. The orilice transmission factor T is 0.01. It is seen that thel clrectivity ratio D is a minimum in the neighborhood of degrees and this is the region that is ordinarily used. There are, however, two other regions, with centers at approximately 60 and 3.00v degrees, which might be used although they are somewhat narrower in angular band width and the minimum values of D are higher than in the preferred region at 180 degrees. At zera and 360 degrees the ratio D becomes unity, which is; the limiting case of no discrimination to the direction of travelA of the' waves in the line 3.

Fig. 5 presents a family of curves showing the directivity` ratio D as a function of the intra-pair phase shift angle with the displacement factor R. as the third parameter. Only the region of most interest, between 168 and 216A degrees, is shown. The orice transmission factor T is 0.01 in each case.. The solid-line curve 20 shows, to a considerably enlarged scale, the central portion of the curve of Fig. 4, for which R is unity. There are two points where D is a minimum, one at 176 and the other at 180 degrees.

The angular band width of the D'-,8 characteristicmay be increased, at the expense of a higher maximum directivity ratio within the band, -by decreasing the` displacement factor R. For example, curve 2I of Fig. 5 shows, the characteristic for an. R ofY 0.9; and curve 22 for an R of 0.8. As the value; of R is reduced from unity it is Seen from the curves: that the left. minimum shifts from 1'76 to 178 degrees. and: stays almost fixed atthat` point, thatthe righ-t minimum moves to the right, to 192 degrees for curve 2I and to 206 degrees for curve 22, thus. widening the band, andL thatr the intermediateV maximum value of D increases inheight, being. approximately 0.0.1 for curve 2 I and 05.04 forcurve 22".

Any lQ-. curve can, of course, be converted to a. directivity-frequencyL curve by assigning a value-to B, the intraepair spacing.V The resulting curve will have the same' general shape as the orignal and the.. position of the. low frequency minimum Willf beY a, direct function. of the intrav-v rarspaeins B..

The formula forthedetermination of the directivitylo: for, the' eght=hole directional' cou pler of Fig. 3, making the. same assumptions as FQlmlllf. 1t. SJ-

and; 6a', 053.1%l and'. T have the same significance as in Formula. 1.

Fig; 6 ia a curve.` showing the variation of the directiyity ratio. Di of an eight-.hole coupler overa complete cycle of the intra-pair phase shift angle /3l when. the displacement factor R is unity and'. the. ori'ce transmission factor T is 0.01'. There; are ten maximal and'- ten minima inthe cycle.: and-i the curveis; very' nearly symmetrical about a Vertical axis at 178 degrees. The region in which lies between 144 and 212 degrees is of greatest interest and this portion of the characteristic is reproduced on an enlarged scale as the solid-line curve 23 of Fig. 7.

In the eight-hole coupler, as the displacement factor R is reduced from unity the first minimum point GH moves to the right, the second minimum G12 moves to approximately 178 degrees and remains xed at that point, the third minimum Gl3 also moves to the right, the fourth minimum Gill moves to the right and out of the region of interest, the r'st maximum HH moves to the right and decreases in height, and the second maximum H12 also moves to the right but increases in height. For all eight-hole couplers in which R is less than unity the second minimum, occurring at 178 degrees, is considered to be the basic one and therefore it is adjusted to fall at the desired frequency by a proper selection of the intra-pair spacing B.

The characteristic corresponding to an R of 0.872 is of particular interest and is shown as the broken-line curve 24 of Fig. 7. It is seen that the rst two maxima II2| and H22 are equal in height and are considerably lower than the maxima Hi I and H|3 for curve 23. For a maximum allowable directivity ratio D equal to the height of the maxima H2! and H22, which isV 0.016, the angular band width for an R of 0.872 is about 39 degrees, compared to a band of about 19 degrees obtainable for an R of unity. In this case the band width has been more than doubled by decreasing R from unity to 0.872 which, therefore, is an optimum value. For a four-hole coupler having the same maximum D in the band, for which R is 0.875, the angular band width is about 25 degrees. Therefore, in this case, increasing the number of holes from four to eight and using the optimum value of 0.872 for R has increased the band width by about 56 per cent.

The D- characteristics shown in Figs. 4 to 7 were computed in accordance with the Formulas 1 and 7. However, measurements made on a number of directional couplers have shown that the actual curves obtained are much flatter over the band and more spread out than the ones shown. The measured curves are found to be in good agreement with the computed curves if the theoretical value of the displacement factor is increased by about ve per cent. Therefore, the correct optimum value of the displacement factor R for the two equal maxima H2! and H22 shown in curve 24 of Fig. 7 is 0.872 plus five per cent, or approximately 0.92. That is, by Equation 4, the displacement A should be equal to approximately 1.84 times the intra-pair spacing B.

What is claimed is:

1. A directional coupler comprising a main transmission line, a longitudinally juxtaposed auxiliary transmission line, and four pairs of coupling links interconnecting said lines at longitudinally spaced intervals, all of said coupling links having substantially equal transmission factors and the ratio of the spacing between the centers of adjacent links of adjoining pairs to twice the spacing between the centers of the links comprising a pair being approximately equal to 0.92.

2. A directional coupler in accordance with claim l in which said auxiliary transmission line is terminated at each end in an impedance substantially equal to its characteristic impedance.

3. A directional coupler in accordance with claim l in which said coupling links have substantially equal phase shifts.

4. A directional coupler in accordance with claim 1 in which said transmission lines have substantially the same phase velocity.

5. A directional coupler in accordance with claim 1 in which said transmission lines are wave guides having a common wall and said coupling links are holes in said common wall.

6. A directional coupler comprising two Wave guides having a common side wall and four pairs of holes in said common wall at longitudinally spaced intervals, said transmission lines having substantially the same phase velocity and the ratio of the spacing between the centers of adjacent holes of adjoining pairs to twice the spacing between the centers of the holes comprising a pair being approximately equal to 0.92.

JOHN T. BANGERT. SIDNEY ROSEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,375,223 Hansen May, 1945 2,408,485 Mason Oct. 1, 1946 2,423,390 Korman July 1, 1947 FOREIGN PATENTS Number Country Date 545,936 Great Britain Sept. 10, 1942 OTHER REFERENCES VProceedings ofthe I. R. E., vol. 35, No. 2, February 1947.

Technique of Microwave Measurements, by Montgomery, Radiation Laboratory Series, Pub. in 1947 by McGraw-Hill. 

